A LAN is a well-known means of achieving communication between different resources, typically computer resources such as computers, work stations, printers and the like. The LAN itself includes a network interface connected to each resource and a physical communication medium connecting all of the interfaces. The interface and connected resource constitute a node. Each node has an unambiguous address or identification (ID). Communication between nodes is typically accomplished by sending and receiving an ordered Group of bits known as a frame or packet. Each frame is sent from a source node, and is received by a destination node. The ID of the source node (SID) and the ID of the destination node (DID) are frequently included within the frame in Groups of sequential bits known as fields. The technique of communicating between the nodes, and of controlling the composition of frames, is defined by a network protocol.
The network protocol includes a MAC aspect which establishes an orderly and predictable ability of each node to access the medium, for the purposes of communicating with another node by transmitting and receiving frames, of requesting access to the medium and acknowledging previous frame communication. Without an orderly and predictable MAC technique, chaotic and inefficient communication, if any, would prevail, because it is highly unlikely that the frames sent from the source node would reach the destination node without interference and disruption caused by conflicting frames sent by another node at the same or overlapping time periods or at a time that the destination node was not ready to receive a frame. Therefore, the MAC facilities are one of the very important aspects of any LAN-like communication protocol among a plurality of equal peer-type transmitting and receiving stations such as nodes.
Because of the increasing recognition of the benefits of communicating information quickly between resources and of sharing resources in computational situations, LANs and networking in general are becoming widely used. Networking of personal computers and work stations allows for easy and effective communication and exchange of information between computers, as well as cost effective sharing of computer resources such as hard disks and printers.
Implementing a LAN can present a significant impediment when it is recognized that all of the resources must me wired together, particularly if the resources are physically separated and numerous. It is not unusual that many thousands or tens of thousands of feet or meters of cable may be required to connect a few tens or hundreds of resources, even when none of the resources is separated by more than a few hundreds of feet or meters. In existing facilities, sufficient physical access may not be available to route the necessary cabling. Installation, even if possible, may be very expensive. Even in designing and constructing new facilities, the cable expense itself for networking among a large number of personal computers or work stations may be cost-prohibitive.
Networks of LAN-like functionality have been established in the past by implementing the communication medium with wireless RF links between the resources. One difficulty presented by such systems is that MAC becomes considerably more difficult, because the RF links do not permit the transmitting and receiving stations (akin to nodes on a LAN) to sense the use of the medium (the RF signals) as reliably as in a wired network. Timing and synchronization requirements for the transmission of messages, static and interference from sources of RF noise, transmission and reception range limitations, multipath interference and fading and other known difficulties, all become significant concerns and limitations in implementing MAC protocols for wireless networks. These same concerns are generally not regarded as highly significant in wired or optical fiber networks because the integrity of the cabled medium usually avoids most if not all of these concerns. The integrity of the wired communication medium usually eliminates or significantly reduces the concerns about interference because the cabling offers inherent shielding from interference. Because the integrity of the communication is essentially assured in transmissions over the wires, range and signalling issues generally do not become significant. Light links have also been employed in networks, but the difficulties with light linked networks are usually even more exaggerated because of the directionality required for directing light beams in unobstructed, line-of-sight, signal paths.
To make the communications more reliable by avoiding many of the problems caused by the difficulties associated with the wireless medium, a variety of different MAC techniques have been employed in wireless network systems. In general the objective of these MAC techniques has been to add reliability to the communication process by compensating, to a certain degree, for the greater uncertainties associated with the wireless medium.
One of the most widely used MAC techniques, originally developed for wireless network systems, but now employed for several of the most common wired network standards, is referred to as carrier-sense multiple access (CSMA). In CSMA, each station uses its receiver to monitor the network medium for other transmission activity prior to activating its transmitter. If any such activity is detected, the station waits until a predetermined time after the end of the detected network activity. If two or more stations begin transmitting at close enough to the same point in time so that none of these stations detect each other's transmission, the resulting transmissions are said to collide, with the result that none of the frames being transmitted by these stations are able to be successfully received at their intended destinations.
While CSMA protocols offer very low latency to begin communication during periods when little or no other network message traffic is active, they perform poorly when many stations are contending for access to the medium to send frames or when the aggregate amount to be transmitted exceeds about half of the data bandwidth of the network medium. In addition to this problem with saturation at well below the rated capacity of the network, wireless CSMA networks have increased chances for collisions when compared with wired CSMA networks, because obstructions to RF signal propagation could permit a station to erroneously detect an available network medium, allowing that station to activate its transmitter while another station was in the process of sending a frame.
Another MAC technique which is typically used in wireless networks is referred to as time division multiple access (TDMA). In TDMA, the available time for the multiplicity of the stations to access and use the radio links is divided among each of the stations. Each station has its own predesignated and assigned time Txop for communicating RF messages with other stations. Each station recognizes and operates under recognition of the order and sequence of the time Txops assigned to the other stations, to avoid overlap and conflict in the communications.
While TDMA protocols are generally very effective in providing reliably recognized opportunities for communicating messages, they can result in a reduced capacity or data bandwidth for transmitting information between stations when the communications are intermittent quantities of variable length messages ("bursts"). In burst message situations, which are highly typical of LAN-type communications, TDMA results in reduced useable data bandwidth because a large portion of the available time is unused for data communications because that time is assigned to stations that have nothing to send when their time slots occur. In situations where one station may have a considerable amount to send, the information must be broken up into parts and sent in sequence, one part each time the station's time occurs. Thus, TDMA, while providing reliable medium access in the difficult medium access environment of wireless networks, does not provide the higher message throughput or bandwidth as do some of the more traditional LANs.
Another MAC technique which is typically used in wireless networks is referred to as packet reservation multiple access (PRMA). In PRMA, each of the multiplicity of the stations must request and reserve a time to access and use the radio link to send its packets or frames. The requests are made on the basis of the amount of time that each station expects to use in communicating the amount of information it has queued for delivery to another station. The available time for all the stations to communicate is divided among each of the stations according to the requests of the stations. The time allocation reserved for each station is communicated to all of the stations, so all of the stations recognize which stations have a time allocation, how long the time allocation is and in what order the stations will receive and use their allocations. After this information is conveyed, each station requesting time uses its reserved time in its assigned order to communicate packets or frames with other stations.
PRMA techniques are more effective than TDMA techniques in utilizing the available time, because only those stations with messages to send need to be provided with an opportunity to send messages. However, fast response to requests is impossible because of the delays inherent in obtaining a reservation. A considerable amount of the available time is consumed in the rather complex communication of control information, referred to as "overhead." The overhead is used for requesting time, allocating a reservation of time, communicating the amount of time reserved, communicating the order in which the stations receive the time reservations, and the like. As a consequence, the quantity of useful data bandwidth of PRMA networks is also limited.
Another recent development in the field of computing is the increasing proliferation of battery-powered, portable computers. These portable computers allow computational tasks to be performed wherever the user happens to be located. Portable computers are usually used during travel, because portability is their primary advantage. Even during travel, however, there may be a need to access other computer resources through the portable computer, just as is more typically done with stationary resources. It may desirable to create temporary, ad hoc networks of portable computers so that, for example, users can network their portable computers to exchange data in meetings and classrooms. Of course in these situations, physically connecting the portable computers to a wired network medium may be inconvenient or impossible. In addition, the users and their locations may not be specifically predefined, and may change intermittently. In addition, to connect portable computers with RF or other wireless networking capability, it is necessary that the transmitters and receivers also operate from battery power, otherwise one of the primary benefits of wireless networking is negated by requiring the use of a power wire instead of a network medium wire. The additional power drain resulting from operating the transmitters and receivers diminishes the available power for the portable computer. If separate batteries are employed for the transmitter and receiver on one hand and for the portable computer on the other hand, the batteries for the transmitter and receiver should be able to provide as much longevity of use for the transmitter and receiver as the batteries for the portable computer provide, without being so large or heavy as to interfere with portability.
A major obstacle to adequate battery life for battery-operated wireless network interfaces is that conventional MAC protocols, whether using CSMA techniques, TDMA techniques, PRMA techniques, or other techniques (such as token passing), all assume that the network receivers are capable of receiving frames at all times that they are not actively transmitting. Consequently these MAC prior techniques are concerned only with controlling access to the network medium by transmitters. Because low-power, short-distance radio transceivers consume about as much electrical power in their receiving function as in their transmitting function, a useful protocol for battery operated networking must avoid this assumption, and must concern itself with the access to the network medium by the receivers as well as the transmitters.
It is against this background that the significant improvements and advancements of the present invention have evolved.